An optical system comprising one or more optical elements such as a projection lens system having a large number of lens elements, i.e. a PO, is used in optical photolithographic projection systems which are known as wafer steppers or as wafer step-and-scanners. Such projection systems are used, for example, for manufacturing integrated circuits, or ICs. In a photolithographic projection system, a mask pattern present in the mask is imaged a large number of times, each time on a different area (IC area) of the substrate by means of a PO with a projection beam having a wavelength of, for example, 365 nm in the UV range or 248 nm in the deep UV range.
One technique for measuring aberrations of an optical system is point-diffraction interferometry (PDI). The PDI is presented and described in articles by R. N. Smartt and J. Strong entitled “Point Diffraction Interferometer” J. Opt. Soc. Amer. 62, p 737 (1972) and by R. N. Smartt and W. H. Steel entitled “Theory And Application Of Point-Diffraction Interferometers,” Japan J. Applied Physics 14, p 351 (1975) as an interferometer belonging to a class of interferometers that measures the variations of phase across a wavefront, is a common-path interferometer, and has the usual advantages of that class. The fringes are very stable against vibration and a white-light source can be used. Although not required for its coherence, a laser is a very useful source for the PDI since it overcomes the rather large loss of light therein. In such interferometers a coherent reference wave, usually a spherical or plane wave, is made to interfere with the wave being examined. The interference shows the variations of phase difference across the wavefronts as variations of the fringe position. The PDI produces its reference wave by diffraction of some of the light at a point discontinuity placed in the path of the beam that is being measured.
The wave being examined by a PDI is brought to a focus to produce an image, usually with aberrations, of a point source from which it came. An absorbing film placed in the focal plane has in it a diffracting point which can be either a small pinhole or a small opaque disk. The wave is transmitted through the film with reduced amplitude and, in addition, some light is diffracted by the diffracting point into a spherical wave. The usual adjustments of an interferometer are possible. A tilt can be produced between the wavefronts, in order to introduce straight fringes, by displacing the diffracting point laterally from the center of the image. A longitudinal displacement out of the focal plane introduces circular fringes.
The PDI is closely related to the phase-contrast test of Zernike in which a small diffraction disk introduces a π/2 phase shift between the corresponding diffracted and non-diffracted beams [see Section 8.5.1 entitled “Zernike Test and Its Relation to the Smartt Interferometer” in Optical Shop Testing, 2nd Edition, D. Malacara, Ed., Wiley (1992)]. The test is used with no tilt and the π/2 phase shift increases the sensitivity to small phase variations by moving the position of zero phase away from the interference maximum. The Zernike phase-contrast test detects changes in an interferogram relative to an interferogram of an aberration free optical system.
The principle of the PDI has been applied to other forms of interferometers to obtain PDIs that are not common path interferometers such as described in U.S. Pat. No. 5,076,695 entitled “Interferometer” by Y. Ichihara and in U.S. Pat. No. 5,548,403 entitled “Phase Shifting Diffraction Interferometer” by G. E. Sommargren. Phase shifting (PS) is introduced in the PDI to create a phase-shifting point-diffraction interferometer (PS/PDI) such as described by Sommargren, supra, to enable the measurement of the interference signal component of the resulting interferograms.
In prior art PDI and PS/PDI, the primary measured quantity is related to the pupil or frequency response function of the optical system and the spatial impulse response or transmission function is not obtained or determined. As a consequence, measurements of the pupil function are made on a surface comprising an image of the pupil function which is displaced from an image plane of the optical system being measured. This feature of the prior art PDI and PS/PDI represents a disadvantage in applications where it is not practical to introduce the diffracting point and subsequent detection system of a PDI or PS/PDI in the image plane of the optical system.
Another disadvantage of PDI and PS/PDI as practiced in prior art is a weak signal because of the high absorption in the mask in order to obtain high fringe visibility.
Other methods used in prior art for detecting effects of certain aberrations of an optical system are based on an intra-field error map for the optical system as described in U.S. Pat. No. 6,906,780 entitled “Method And Apparatus For Self-Referenced Dynamic Step And Scan Intra-Field Lens Distortion” by A. Smith or based on the measurement of relative displacements of images of artifacts formed by an imaging system such as described in U.S. Pat. No. 6,963,390 B1 entitled “In-Situ Interferometer Arrangement” by A. H. Smith and R. O. Hunter, Jr.
A yet another method used in the prior art for detecting aberrations of an optical system comprises the steps of:                arranging a test object in the object plane of the optical system;        providing a resist layer in the image plane of the optical system;        imaging the test object by means of the optical system and an imaging beam;        developing the resist layer; and        detecting ex situ the developed image by means of a scanning detection device having a resolution which may be comparable to or considerably larger than that of the optical system.        
When the resolution of the scanning detection device is considerably larger than that of the optical system, the detection device allows observation of details which are considerably smaller than the details generated by the optical system.
The method of the prior art described above is known for example from EP 0 849 638 A2 by K. Kaise, T. Tsukakoshi, and T. Hayashi and U.S. Pat. No. 6,331,368 B2 by P. Dirksen and C. A. H. Juffermans relating to methods for measuring the in situ aberrations of the optical system in lithographic projection apparatuses.
Another method for the measurement of properties of a PO is described by P. Dirksen, J. J. M. Braat, A. J. E. M. Janssen, Ad Leeuwestein, T. Matsuyama, and T. Noda in a paper SPIE, 6254-34, San Jose, Feb. 22, 2006. The paper which is entitled “Aerial image based lens metrology for wafer steppers” discusses an alternative lens metrology method that is based on an aerial image measurement and compares the alternative lens metrology method to a method based on phase measurement interferometers.
The aim of a photolithographic projection system is to integrate an ever-increasing number of electronic components in an IC. To realize this, it is desirable to increase the surface area of an IC and to decrease the size of the components. For the optical system, this means that both the image field and the resolution must be increased so that increasingly smaller details, or line widths, can be imaged in a well-defined way in an increasingly larger image field. This requires an optical system which must comply with very stringent quality requirements. Despite the great care with which such an optical system has been designed and the great extent of accuracy with which the system is manufactured, such a system may still exhibit aberrations such as spherical aberration, coma, and astigmatism and flare which are not admissible for the envisaged application. In practice, a lithographic optical system is thus not an ideal, diffraction-limited system but an aberration and background-limited system.
The aberrations are dependent on the positions in the image field and are an important source of variations of the imaged line widths occurring across the image field. When novel techniques are used to enhance the resolving power or the resolution of a lithographic optical system, such as the use of phase-shifting masks as described in, for example, U.S. Pat. No. 5,217,831 or when applying an off-axis illumination as described in, for example, U.S. Pat. No. 5,367,404, the influence of the aberrations on the imaged line widths is still an important source of variation.
Moreover, the aberrations of the optical system are not constant with respect to time in modern lithographic. To minimize low-order aberrations, such as distortion, curvature of the field, astigmatism, coma, and spherical aberration, these systems comprise one or more movable lens elements. The wavelength of the projection beam or the height of the mask table may be adjustable for the same purpose. When these adjusting facilities are used, other aberrations may be introduced. Moreover, since the intensity of the projection beam must be as large as possible, a lithographic optical system is subject to aging over extended time periods so that the extent of the aberrations may change with respect to time.
The performance of the optical system has also been shown to be dependent on the amount of energy absorbed during the exposure of wafers and as a result varies over time periods as short as of the order of the time to expose a wafer. Such heating effects are described, for example, in the article entitled “Fine Tune Lens Heating Induced Focus Drift with Different Process and Illumination Settings” by Y. Cui, Optical Lithography XIV, C. J. Progler, Editor, Proceedings of SPIE Vol. 4346 (2001) and the article entitled “Correcting Lens Heating Induced Focus Error”, ASM Lithography-Application Bulletin 4022-502-95041, p 2 (1996).
It has also been proposed to use for the projection beam a beam of extreme UV (EUV) radiation, i.e. radiation at a wavelength in the range of several nm to several tens of nm. The resolution of the optical system can thereby be enhanced considerably without increasing the numerical aperture (NA) of the system. Since no suitable lens material is available for EUV radiation, a mirror projection system instead of a lens projection system must then be used. A lithographic mirror optical system is described in, for example, EP 0 779 528 by D. M. Williamson. For reasons analogous to those for the lens projection system, there is a need for an accurate and reliable method of measuring in situ aberrations for this EUV mirror optical system as well.
The speed or throughput of a method used to measure in situ aberrations may also limit the utility of the method. Low throughputs are generally associated with methods based on the ex situ measurement of developed images of a test mask formed in the resist layer. Also low throughput is associated with ex situ measurements when the developed image is scanned with a scanning detection device, e.g. a SEM such as described in an article entitled “Application Of The Aberration Ring Test (ARTEMIS™) To Determine Lens Quality And Predict Its Lithographic Performance” by M. Moers, H. van der Laan, M. Zellenrath, Wim de Boeij, N. Beaudry, K. D. Cummings, A. van Zwol, A. Becht, and R. Willekers in Optical Microlithography XIV, C. J. Progler, Ed., Proceedings of SPIE Vol. 4346 (2001),p 1379 and in cited U.S. Pat. No. 6,331,368 B2.
In cited EP 0 849 638, it is proposed to detect ex situ the developed image with optical means to address the laborious work otherwise required by technology that uses a complicated microscope such as a SEM. To this end, a test mask having one or more patterns of strips which are alternately radiation-transmissive and radiation-obstructive, i.e. an amplitude structure, is used. The comatic aberration of a projection system can be detected with such a pattern. The detection is based on measuring the widths of the light or dark strips in the image formed and/or measuring the asymmetry between the strips at the ends of the image of the patterns.
In prior art wherein measurements are made of developed images, it must be recognized that the development of “latent images” in resist is a highly nonlinear process which can limit the utility of a method based on ex situ measurement of in situ aberrations. In particular, the nonlinear process converts three-dimensional topographic information contained in a latent image in undeveloped resist into two-dimensional shapes in developed resist in the plane of the wafer.
It is evident from the considerations above that there is an increasing need for a reliable and accurate method with a high throughput for in situ and ex-situ measurement of in situ spatial impulse response function, changes in optic axis location, and image plane location of the imaging system over short and long time periods.